There is disclosed in the above cited copending patent application Ser. No. 496,743 a standing crop harvester which has a chassis adapted to be displaced along the ground in the transport direction and carrying a pair of guide wheels which are normally controlled from the steering wheel. A sensor carried on an arm in front of the machine detects the edge of the swath in a standing crop and generates an output which varies the impedance of one element of a bridge circuit. Another element of this bridge circuit has its impedance varied in accordance with the inclination of the harvester on the ground, another element is varied in accordance with the speed of the harvester on the ground, and a fourth element is varied in accordance with the position of the wheels of the harvester relative to the direction of travel. A zero-voltage detector connected across this bridge operates an electromechanical valve which steers the vehicle automatically, compensating for inclination of the ground under the harvester and speed of the harvester along the ground, and using the feedback signal from the wheel position so as accurately to guide the harvester along the edge of the swath. The steering wheel is provided with an electrical capacitive sensor which shuts off the automatic steering system immediately when the operator touches the wheel so that he can readily take over the manual operation of the harvester. In addition a double check valve is provided between the power steering system and the hydraulic cylinder serving to guide the wheel of the vehicle so as to prevent rotation of the steering wheel when the automatic pilot is in operation.
The above-cited copending application Ser. No. 421,494 describes an agricultural machine having a pivotally mounted implement support which is vertically positioned by a valve-operated hydraulic piston-and-cylinder arrangement. A single control lever is operable from the central hold position between two nonlocking and nonindexed raise and lower positions to operate this piston-and-cylinder arrangement as well as to locking and indexed follow and set position. In the set position the switches carried by a set-point indicator are connected to the hydraulic-cylinder valve, and to an actual value indicator pivoted next to the set-point indicator on the frame of the machine and linked to the support serves to operate these switches to maintain the support in a predetermined position relative to the machine frame. In the follow position the support can be positioned in a mode whereupon the pressure in its ram is kept within a certain range so that this support rides lightly on the ground or limit switches operated by a ground can be connected through the control to the hydraulic-cylinder valve to maintain the support at a predetermined spacing above the ground.
It is a difficulty with automatic steering systems that when a gap in the standing crop is encountered the apparatus tends to hunt around to follow the contours of the gap. This is disadvantageous in that it is a frequent occurrence, especially with a row crop such as corn or soy beans, that several plants will be missing in a row. The principal disadvantage of this hunting in order to follow the contours of the gap in the crop is that the irregular swath cut during the hunting is then followed in each successive swath so that the harvesting operation, which normally follows a regular or straight path along the rows of the crop, becomes irregular.
Such automatic steering systems usually employ a crop sensor in the form of complicated lever-potientometer arrangements that feel the standing crop and generate an output proportional to the proximity of a row of the crop to the detector. These devices are relatively expensive and tend to foul so that they must frequently be serviced in order to function properly.